1. Field of the Invention
This invention relates in general to an improved gas-liquid contacting tray of the type used in distillation and absorption systems for mass transfer between two fluids, and more particularly to improved means for promoting complete tray activity over the entire tray surface.
2. Description of the Prior Art
In the art of mass transfer for selectively separating at least one component from a mixture of at least two constituents as for example in distillation and absorption applications, an upwardly flowing vapor or gas stream is typically contacted on a substantially horizontally aligned contacting surface with a generally downwardly flowing liquid stream. In a conventional distillation process, such contacting permits the upwardly flowing vapor or gas stream to become selectively enriched with the lighter components of the mixture, i.e., those components with relatively high volatilities, while the generally downwardly flowing liquid stream becomes selectively enriched with the heavier component of relatively low volatilities.
A variety of types of gas-liquid contacting trays have been conventionally employed in the above described mass transfer operations, to effect intimate contacting between the respective gas and liquid phases. The degree of mass transfer of a given component between these phases is determined largely by the intimacy of this contact. Maximum utilization of the gas-liquid contacting tray requires a close approach to phase equilibrium at all points on the tray, but this condition is difficult to achieve in many instances with trays of the prior art.
A major factor which has prevented reasonable approaches to phase equilibrium has been the existence of partial inactivity on the gas-liquid contacting surface whereby the process vapor fails to pass through the prior art tray and the liquid thereupon at all points uniformly. Under such conditions, a substantial portion of the contacting surface merely transports clear liquid and does not contribute to the mass transfer process. Furthermore, such inactive areas are prone to weep or drain liquid onto the tray below thereby altering liquid-vapor ratios on one tray and upsetting fluid composition on the next tray.
Partial inactivity occurs on a given tray when the tray and its liquid present nonuniform resistance to vapor flow entering the tray. A common cause is the hydrostatic gradient which normally comprises the driving force for moving liquid across the tray. Inactivity can be reduced significantly by eliminating this gradient, for example, by employment of vapor thrust, rather than gravity, to propel the liquid. Vapor thrust for this purpose may be generated by inclined openings properly arranged to move the liquid at desired velocity and in a desired direction, as is well known to those skilled in the art.
Despite the elimination of gradient and other improvements of the hydraulic behavior of the gas-liquid contact-tray, it is well known that a certain area, notably the liquid inlet area of the tray, exhibits chronic inactivity, particularly in large diameter trays operating with a low vapor pressure drop across the tray.
The prior art has attempted to overcome the inlet inactivity problem on gas-liquid contacting trays by means of various types of devices, commonly referred to as "bubble promoters," disposed at the inlet region of the liquid flow path across the tray surface to positively initiate bubbling activity. Such bubble promoter devices generally operate by increasing momentarily the velocity of the liquid at the tray inlet region, and thereby reducing its hydrostatic head. Bubbling is initiated immediately at the tray threshold and, once initiated, continues across the tray surface.
One type of inlet bubbling promoter means employed in prior art trays is disclosed in U.S. Pat. No. 3,282,576 to W. Bruckert et al. The Bruckert promoter comprises an upwardly and inwardly extending imperforate wall integral with the imperforate liquid receiving pan of the tray, and a downwardly and inwardly sloped perforated wall connected at one end to the top edge of the upwardly extending imperforate wall and at its other end to the gas-liquid contacting member or tray deck. Liquid introduced from a previous tray by downcomer means impinges on the imperforate receiving pan and passes over the upwardly extending wall and thereafter flows downwardly to the tray deck over the inwardly sloped surface. Inlet bubbling is achieved by the reduction in the hydrostatic liquid head on the downwardly sloped surface relative to the remainder of the tray surface. This bubble promotion device thus has an active bubbling surface which is constructed as a descending ramp and, expectedly, liquid flowing down the ramp surface experiences a gravitational acceleration. This acceleration increases the liquid's entrance velocity as it enters onto the tray deck, a condition which in some instances may be detrimental to hydraulic behavior of the tray, giving rise to liquid maldistribution on the gas-liquid contacting surface.
Another type of bubbling promoter is disclosed in the prior art which relies on a somewhat different phenomenon to reduce the liquid's resistance to gas flow at the tray inlet region. This promoter comprises a vertically extending wall member which separates the imperforate liquid receiving pan from the active gas-liquid contacting surface, with a perforated plate member connected to the vertically extending wall member at its upper end, so that the perforated member extends laterally and inwardly from the liquid inlet region, above a portion of the gas-liquid contacting surface. Liquid introduced from a previous tray via downcomer means impinges on the imperforate receiving pan and then flows over the vertical wall member, across the perforated plate member and drops onto the tray deck. Experimental testing has shown that the major portion of inlet bubbling resulting from the use of this bubble promoter is provided by gas which passes through the tray deck under the horizontally extending perforated plate member and exits through the space between the downstream edge of the horizontally extending perforated plate member and the tray deck, while only an insignificant portion of the bubbling is provided by gas passing in series through the tray deck and through the perforated plate member of the promoter. By initiating bubbling in this fashion, not only is the liquid discharged from the downstream edge of the perforated plate excessively aerated but this liquid is also thrown across the tray deck by the gas issuing from the aforementioned space between the tray deck and the perforated plate member downstream edge.
Still another bubble promotion device which has been employed by the prior art comprises an upwardly and inwardly slanted imperforate plate which separates the imperforate liquid receiving pan of the tray from the active tray deck thereof, with a vertically extending wall member connected to the active tray deck below the downstream edge of the slanted plate, terminating at its upward extent just below the downstream edge of the slanted plate. This construction forms a continuous slot opening across the tray inlet which is in gas flow communication with the vapor space of the next lower tray. Liquid discharged from the previous tray in the distillation column flows via downcomer means to the imperforate receiving pan of the tray, flows upwardly along the slanted plate and falls from the downstream edge of the slanted plate onto the tray deck. The flowing liquid is contacted with the upwardly flowing gas stream which issues through the slot opening of the bubble promoter to produce a gas-liquid biphase. Published test results show that the gas-liquid froth profile across the tray which is achieved by this upwardly slanted plate bubble promoter is similar to that achieved on the previously-described horizontal perforated plate bubble promoter, indicating that the same inlet bubbling activation phenomenon, together with the associated disadvantages of excessive aeration at the tray inlet and "throw" of liquid downstream across the tray deck, are present in both cases.
The operation of the above-described prior art bubble promoters, as indicated, involves a propulsion of the liquid onto and across the gas-liquid contacting surface, or active tray deck. While such liquid propulsion may favorably reduce the hydraulic gradient on many gas-liquid contacting trays of the cross flow sieve type, this propulsion on trays utilizing vapor directing orifices is generally disadvantageous. Sieve trays are gas-liquid contacting trays employing a more or less uniform pattern of fixed size perforation openings with walls normal to the tray surface distributed over the gas-liquid contacting surface. It is well established that the performance of sieve trays may be improved by the employment of vapor directing orifices, as for example of a type disclosed and claimed in U.S. Pat. No. 3,417,975 to B. Williams et al., on the gas-liquid contacting surface. Trays designed in accordance with the Williams et al patent (hereinafter referred to as slotted sieve trays) perform with improved efficiency owing to the elimination of longitudinal hydraulic gradient in the liquid flowing across the tray. In such slotted sieve trays, the use of the afore described prior art bubble promotion devices tends to yield acceleration of the liquid across the tray contacting surface. This acceleration of the liquid limits the overall mass transfer efficiency which is achievable by the tray, by significantly reducing the residence time of the accelerated liquid on the tray, particularly at the centerline region thereof.
Another problem associated with the above-described prior art bubble promoters is their tendency to produce spraying in the inlet region of the tray. Such spraying is vastly different from the desired normal condition of high gas-liquid froth height on the tray surface. The desired froth comprises bubbles of gas more or less homogeniously interspersed in matrix of liquid, whereas a spray comprises discreet drops of liquid dispersed in the gas space above the tray surface. A high froth height on the tray surface is generally associated with a high tray contacting efficiency, assuming that the froth height is also uniform over the entire tray contacting surface, while a high spray height is generally detrimental to tray performance and is associated with poor tray contacting efficiency. The poor contacting efficiency associated with the spray can be partially attributed to the change in the nature of the gas-liquid contact from the efficient liquid phase continuous regime characterizing a froth to the inefficient gas phase continuous regime characterizing a spray. Additional inefficiency results from the excessive aeration of liquid in the spray region, yielding a disproportionate amount of flow through in this area of the tray relative to the more normally behaving regions of the tray. An important consequence of spraying above the gas-liquid contacting surface of the tray is the resultant tendency toward entrainment of the liquid in the gas passed upwardly through the distillation column. Such entrainment is severely detrimental to overall process efficiency and may lead to premature flooding of the column. Under such conditions, flooding can only be avoided by the comparatively expensive remedy of increasing the tray spacing in the column. High levels of entrainment caused by excessive spraying results in further reduced tray efficiency by virture of recycling of a portion of the already distilled liquid from a given tray back to the gas-liquid contacting zone associated with the previous tray, where the degree of separation is reduced substantially different.
In view of the foregoing deficiencies of the respective prior art bubble promoting devices, one of ordinary skill might reasonably propose to avoid the gravitational acceleration of liquid as associated with the Bruckert bubbling promoter, and to avoid the high liquid propulsion and spray levels associated with the upwardly slanted plate bubble promoter, by various structural modifications of the horizontally extending perforated plate member 66 promoter. To reduce the inlet spraying level associated with the horizontal perforated plate bubble promoter, for example, one may propose to reduce the number of perforations per unit area in the horizontal plate to a comparatively low level. Separately, or in conjunction with this change, it may also be proposed to reduce the length of the perforated plate in the horizontal direction to lower the level of inlet spraying as well as decreasing acceleration of the liquid across the tray. It may further be proposed to reduce the height or elevation of the perforated plate to reduce the level of inlet spraying and liquid acceleration. However, none of these changes, either alone or in combination, are able to reduce liquid propulsion to an acceptable level, although some reduction in inlet spraying may be achieved.
Accordingly, it is an object of the present invention to provide a gas-liquid contacting tray with improved inlet bubbling means.
It is another object of the invention to provide a tray with bubbling means which adequately initiates inlet activity while substantially eliminating any liquid acceleration effect and inlet spraying, so that a uniform froth height is provided over the entire tray surface.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.